The coordinate bond formed between the donor atom and the acceptor atom generates a positive (partial) charge in the donor group and a negative (partial) charge in the acceptor group:
                              δ          +                ⁢                                  ⁢                  δ          -                                        [                              donor            ⁢                                                  ⁢            group                    →                      acceptor            ⁢                                                  ⁢            group                          ]            
Metallocenes as π complexes and their use as catalysts in the polymerization of olefins have been known for a long time from EP-A-129 368 and the references cited therein. It is also known from EP-A-129 368 that metallocenes in combination with aluminum alkyl/water as cocatalysts are effective systems for the polymerization of ethylene (for example, about 1 mol of trimethylaluminum and 1 mol of water forms methylaluminoxane=MAO). Other stoichiometric ratios have also been employed successfully (WO 94/20506). Metallocenes whose cyclopentadienyl skeletons are covalently bound to one another by means of a bridge are also known.
As an example of the numerous patents and patent applications in this field, mention may be made of EP-A 704 461 in which the bridging group mentioned is a (substituted) methylene group or ethylene group, a silylene group, a substituted silylene group, a substituted germylene group or a substituted phosphine group. In EP-A 704 461, too, bridged metallocenes are envisaged as polymerization catalysts for olefins.
Catalysts having a donor-acceptor interaction and their use as polymerization catalysts are known in principle.
WO-A-98/01455 describes compounds in which a transition metal is complexed by two π systems, in particular aromatic π systems (metallocenes), and the two systems are reversibly joined to one another by means of at least one bridge containing a donor and an acceptor, with the donor and acceptor atoms being bound as substituents to the π systems, and also provides for their use as polymerization catalysts.
WO-A-98/45339 describes compounds in which a transition metal is complexed by two π systems, in particular aromatic π systems (metallocenes), and the two systems are reversibly joined to one another by means of at least one bridge containing a donor and an acceptor, with at least one of the donor or acceptor atoms being part of the associated π system, and also provides for their use as polymerization catalysts.
The patent applications WO-A-98/01483, WO-A-98/01484, WO-A-98/01485, WO-A-98/01486, WO-A-98/01487, WO-A-99/32532 and EP-B1 042 336 describe industrial polymerization processes using the described catalysts having a donor-acceptor interaction.
It is known from these documents that the catalysts having a donor-acceptor interaction can advantageously be used as catalysts for olefin polymerization.
It is not known in the art that metallocenes which have a donor-acceptor interaction can make it possible to carry out polymerizations at elevated temperatures (≧40° C.) so as to give polyolefin elastomers having very high molar masses, and at the same time display a high activity in the presence of small amounts of cocatalyst.